Reliable component operation within computer systems is often hampered by noise that exists on bus lines within the system. FIG. 1 illustrates a typical bus line signal 10 and the noise associated with bus line signal 10 compared to a second signal 13 that does not exhibit noise. High frequency noise may result in inadvertent switching in digital circuits where the noise amplitude is large enough to cross digital voltage thresholds.
A standard, prior art method of eliminating noise employs an RC low-pass filter network with a buffer to form an input buffer circuit 22 as shown in FIG. 2. Input buffer circuit 22 includes an input 12 connected to a resistor 14 which is connected at node A to both a capacitor 16 and a buffer 18 possessing hysteresis. The circuit output 20 is the output of buffer 18. Resistor 14 and capacitor 16, in this configuration, form an RC low-pass filter which is well known by those skilled in the art of circuit design. Circuit 22 effectively filters out high frequency noise, however, it suffers from switching speed limitations. Since node A must charge and discharge at a rate dependent upon the RC time constant formed by resistor 14 and capacitor 16, switching of output 20 is limited. As switching speed increases, node A may not fully charge up to the input voltage when input 12 transitions from a low-to-high value. When input 12 then switches low, the voltage at node A is at an unpredictable voltage and inconsistent buffer propagation delays result. "Recovery time", in this instance, describes the time needed for node A to fully charge and is readily seen in FIG. 3. In FIG. 3, node A still has not fully charged to 3 V 17 nanoseconds after input 12 has switched to 3 V . A recovery time of over 30 nanoseconds is common in prior art circuit 22. Additionally, node A may not fully discharge when input 12 transitions from a high-to-low value. When input 12 switches high again, the voltage at node A is at an unpredictable voltage and further inconsistent buffer propagation delays result. "Recovery time", in this instance, describes the time needed for node A to fully discharge.
Therefore, as one attempts to maximize filtering capability by increasing the circuit RC time constant, the recovery time needed increases, thus further hampering switching speed. Conversely, decreasing the RC time constant of circuit 22 to decrease the recovery time results in a degradation of filtering capability. Prior art filtering solutions, therefore, suffer since filtering quality and fast switching capability tend to be mutually exclusive.
It is an object of this invention to provide an improved filtering circuit that enables high filtering quality with decreased recovery time. It is a further object of the invention to provide effective noise filtering in high speed computer applications. Other objects and advantages of the invention will become apparent to those of ordinary skill in the art having reference to the following specification together with the drawings herein.